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Abstract

Herbivores play a critical role in structuring benthic communities on tropical coral dominated reefs by removing macroalgae. Reducing herbivory has been implicated in promoting phase shifts from coral dominance to other ecosystem states following disturbances. Turbidity and sedimentation are key physical processes that also structure coral reef communities because they limit light penetration and interfere with biological processes such as heterotrophy. Singapore’s coral reefs have been heavily impacted by human activities for decades and experience very high levels of sedimentation and turbidity. As a result, coral and algal growth is restricted to shallow reef flats and upper reef slopes. While macroalgae, particularly Sargassum spp., are abundant on the reef flats (0-2 m depth), adjacent upper reef slopes (3-4 m depth) are dominated by diverse hard coral assemblages composed primarily of sediment tolerant taxa. To gain a better understanding of the role of herbivore biomass and herbivory rates in structuring these disturbed reefs, we examined the relationship between cover of corals, macroalgae, fish herbivore biomass and urchin abundance at eight representative sites among Singapore’s southern islands during two seasons. In addition we filmed herbivory assays on replicate experimental macroalgal stands (4 separate macroalgal species) and natural EAM patches at three of these sites to estimate species-specific rates of herbivory.Average coral cover at 3-4 m depth was surprisingly high (~40%), compared to current levels found on other Indo-Pacific reefs, although comparisons with historical data from these sites suggests ~15% decline in cover over the last three decades. The average biomass of herbivorous fishes (~4 g m-2), on the other hand, was more typical of degraded and heavily overfished reefs, while urchin abundances were within the range found at other Indo-Pacific sites. Herbivorous fish were not observed feeding on macroalgal assays, while fish bites recorded on EAM were mostly made by territorial damselfish. There was a significant but weak relationship between coral cover and fish herbivore biomass but none for macroalgae or between urchin abundance and benthic cover of corals and macroalgae. A possible explanation for the relatively low cover of macroalgae and low biomass of fish herbivores is that a disproportionate amount of the grazing is being carried out by one or a few fish species not captured by visual survey methods. In addition, the composition of the coral community, which is dominated by sediment tolerant taxa that are relatively resistant to thermal stress, has undoubtedly contributed to the ability of these shallow turbid reefs to persist in a coral dominated state. Furthermore, a combination of limited space for algal growth and low light levels at 3-4 m depth may limit algal growth rates so that only low levels of herbivory are necessary to prevent seaweeds outcompeting corals on these disturbed reefs.

Author Comment

This is a revised version of the original submission and has been submitted for peer review to PeerJ. It includes a new table comparing coral cover changes between 1987 and 2012. The discussion has also been edited to discuss the possibility that 1) a disproportionate amount of grazing may be carried out by one or a few species of fish not captured by visual surveys and 2) high coral cover and hence limited space for algal growth may mean that low rates of herbivory are necessary to maintain low macroalgal cover.

Karenne Tun contributed reagents/materials/analysis tools, wrote the paper, reviewed drafts of the paper.

Peter D Steinberg conceived and designed the experiments, performed the experiments, contributed reagents/materials/analysis tools, wrote the paper, reviewed drafts of the paper.

Field Study Permissions

The following information was supplied relating to field study approvals (i.e., approving body and any reference numbers):

All research carried out abided by local laws and was done with permission of the Singapore National Parks Board (Permit no. NP/RP11-089).

Data Deposition

The following information was supplied regarding data availability:

The raw data has been supplied as a Supplemental Dataset.

Funding

This work was funded by the Nanyang Technological University project: “Development of the Advanced Environmental Biotechnology Centre (AEBC)” under the Research Centre Funding Scheme (RCFS), project No. COY-15-EWI-RCFS/N190-2. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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